In an internal combustion engine provided with an exhaust purification system using a catalyst in general, in order to efficiently perform purification of a harmful component in an exhaust gas by a catalyst, control of a mixing ratio between air and fuel in a mixture to be burned in an internal combustion engine, that is, an air-fuel ratio, is indispensable. An air-fuel ratio sensor is provided in an exhaust passage of the internal combustion engine in order to control such an air-fuel ratio, and the air-fuel ratio detected thereby is subjected to feedback control so as to match a predetermined target air-fuel ratio.
On the other hand, in a multiple-cylinder internal combustion engine, since the air-fuel ratio control is usually executed by using the same control amount for all the cylinders, even if the air-fuel ratio control is executed, the actual air-fuel ratio might be dispersed among the cylinders. In such a case, if the degree of the dispersion is small, it can be absorbed by the air-fuel ratio feedback control, and a harmful component in the exhaust gas can be also purified by a catalyst, and thus, exhaust emission is not influenced and a particular problem does not occur.
However, if the air-fuel ratio between cylinders are largely dispersed due to, for example, a failure in a fuel injection system of some cylinders, the exhaust emission is deteriorated, which causes a problem. Such a large degree of dispersion in the air-fuel ratio that deteriorates the exhaust emission is preferably detected as abnormality. Particularly, in the case of an internal combustion engine for an automobile, in order to prevent driving of a vehicle with deteriorated exhaust emission, detection of abnormality in dispersion of the air-fuel ratio between cylinders onboard is in demand, and recently, there is also a movement to legislate this detection.
In the device described in Patent Literature 1 for example, a value of dispersion ratio, which is a degree of dispersion in an air-fuel ratio between cylinders, is acquired from a trajectory length or a trajectory area of an output of an air-fuel ratio sensor by using a map or a function determined in advance.